Stereotactic radiosurgery provides a dose of radiation in a target volume in a patient. The target is irradiated at a multiplicity of orientations with finely collimated beams.
The use of stereotactic radiosurgery to render tissue necrotic is well established and various systems are currently used for stereotactic radiosurgery. The prior art recognizes the need to confine radiation as much as possible to the target volume being treated. Generation of a desired dose pattern at the target volume is the objective of a treatment plan which takes into account limitations of the particular radiosurgical system used. Typical types include a Gamma Unit which utilizes a multiplicity of Cobalt-60 sources arranged on a spherical surface, a linear accelerator (LINAC) which utilizes a photon beam source mounted on a rotating gantry, and a stationary generator for a beam of charged particles. These radiosurgical systems, as well as associated methods, characteristics and performances are described in various publications, e.g., in Stereotactic Radiosurgery, Alexander E. et al., McGraw-Hill, 1993; and in Neurosurgery Clinics of North America, vol. 3, no. 1, Lunsford L. D. (editor), W. B. Saunders Co., Jan. 1992.
Treatment planning capabilities include a selection of a dose level to the target, a choice of collimators for beam shaping and a determination of beam orientations at which radiation is applied to the target volume. In order to reduce the dose deposited in healthy tissue outside the target volume, it is generally desirable to spread beam orientations over a wide range.
The prior art describes beam orientations of the Gamma Unit as being fixed relative to the stationary unit. The orientation of the beam with respect to the target may be determined only by selecting the initial elevation angle of the patient's head relative to the unit. Radiation intensities and exposure times of all unplugged beams are identical for all orientations. Dose patterns may be shaped only by the elimination of selected beams through plugging the corresponding collimators prior to treatment.
A typical implementation of LINAC scanning involves rotating the LINAC gantry about its horizontal axis which orthogonally intersects the beam and the target volume at an isocenter. Such rotation causes a beam of radiation to trace an arc on a sphere surrounding the target. A multiplicity of non co-planar arcs is produced by consecutive gantry rotations, each one associated with an increment of the azimuthal orientation of the patient. The number of arcs is typically between 4 and 11. The beam intensity for each arc stays constant throughout the continuous arc irradiation. The heavy rotating gantry is associated with added expense and reduced accuracy.
Another prior art implementation of LINAC beam scanning is by rotating the patient about a vertical axis which intersects the target and irradiating with a beam which is angled with respect to the axis. Each such rotation is geometrically equivalent to the beam forming a conical surface of radiation. Incrementing the beam slant angle between consecutive rotations produces a multiplicity of coaxial conical radiation surfaces having the target in the focus. Here too, the beam intensity for each conical scan stays constant throughout the irradiation.
Yet another prior art implementation of LINAC beam scanning is by simultaneous rotation of the LINAC gantry and of the patient turntable. The orthogonal axes of rotation intersect the radiation beam at an isocenter coinciding with the target. The rotational speed about each axis is constant. The beam intensity, however, remains constant throughout the continuous irradiation. The heavy rotating gantry is associated with added expense and reduced accuracy.
Charged particles stereotactic radiosurgery uses a different approach. Since the horizontal radiation beam is stationary, beam orientations are obtained by incrementing the azimuthal orientation of the patient as well as the roll angle of the patient about a longitudinal horizontal axis. The axes of rotation intersect the radiation beam orthogonally at an isocenter coinciding with the target. Irradiation is discrete (i.e. non-continuous) from a small number of orientations. Time consuming positioning of the patient is required for each such orientation, thus preventing the use of a large number of orientations.